I believe there are products you can buy for this. If I remember correctly, it was baking soda along with the detergent. I used to use it on my rugs when I had multiple cats in the house and one of them would get in a snit and go on my rigs.

I'd be careful about where you keep your bag from now on. In my experience, when a cat goes somewhere like that, they tend to seek it out and repeat it. Perhaps because the scent never really goes away.

Also, when they do something out of the ordinary like that, it often signals a problem. Perhaps a bladder infection, perhaps they're being finicky about how clean the litter box was.

She hasn't done it since that one time, and she was in heat when it happened. And I don't know if it matters, but perhaps she smelled a friend's cat who took a bit of an interest on it about a year ago. This gym bag by the way, rarely gets used. She (my friend's cat) didn't do anything bad on it, just slept on it.